Inter-terminal capacitances of a three-terminal semiconductor device such as a transistor, an FET, a power MOS FET, and an IGBT are represented, for example, as Cgs, Cds, and Cgd for the case of the FET and described on a datasheet of the device, and can be represented as a delta connection circuit (delta circuit) including three terminals (refer to FIG. 9 to be described later).
On the other hand, generally, electrostatic capacitances of a semiconductor device and the like can be measured through use of an LCR meter, which is also known as an impedance meter. The LCR meter generally measures an AC signal between two terminals, and when the three-terminal semiconductor device including three capacitances connected as the delta configuration is measured, as illustrated in FIGS. 5-29(a) and 5-29(b) of “Agilent Impedance Measurement Handbook, A guide to measurement technology and techniques, 4th Edition”, Agilent Technologies, Inc., Sep. 10, 2013, it is known that high-side and low-side measurement terminals of the LCR meter are connected to two terminals out of the three terminals of the semiconductor device, and an AC guard of the LCR meter is connected to the remaining one terminal for measurement. It should be noted that the AC guard connection is briefly described in Section 2.4.7 on page 2-14 in “Agilent Impedance Measurement Handbook, A guide to measurement technology and techniques, 4th Edition”.
FIG. 9 illustrates a measurement circuit when a measurement circuit using an LCR meter of four-terminal connection illustrated in FIG. 5-29(b) of “Agilent Impedance Measurement Handbook, A guide to measurement technology and techniques 4th Edition” is applied to a case in which an LCR meter of two-terminal connection is used for measuring a MOS FET.
In FIG. 9, in a measurement circuit 900, as a device under test (DUT), a MOS FET 106 including three terminals of a drain (D), a gate (G), and a source (S) is illustrated. A high-side terminal H of an LCR meter 102 of three-terminal connection is connected to the terminal D of the DUT via a cable 902, a low-side terminal L of the LCR meter 102 is connected to the terminal G via a cable 904, and a guard terminal GD of the LCR meter 102 is connected to the terminal S via a cable 906. A voltage between both ends of a capacitance Cgd is measured as a voltage Vm by a voltmeter 110, and a current flowing through the capacitance Cgd is measured as a current Im by an ammeter 114. A current Ie flowing through a capacitance Cds does not flow through the ammeter 114 due to the use of the terminal GD, and the current and the voltage relating to the capacitance Cgd can thus be precisely measured. It should be noted that, as illustrated in FIG. 9, the LCR meter 102 includes a signal source 108 and a ground point 112. Moreover, the virtual capacitances Cgs, Cds, and Cgd are connected in a delta configuration, and exist among the terminals of the DUT 106.
For an apparatus manufacturer which designs a circuit having a three-terminal device built therein, it is important to measure the inter-terminal capacitances of this device on an actual device, and to compare the measured capacitances with parameters on a datasheet, thereby evaluating a performance of the device.
Incidentally, for the measurement of the inter-terminal capacitances of the three-terminal device, as power devices and the like such as a power MOS FET, an IGBT, and the like have developed in recent years, bias voltages to be applied to the respective terminals increase regarding the inter-terminal capacitances. Thus, such an inconvenience that those bias voltages cannot be covered by bias voltages of the related-art LCR meter occurs, and hence there are increasing numbers of cases in which the measurement cannot be easily carried out.